1. Field of the Invention
This invention relates to a method of producing treated silica filler for silicone rubber, and to the filler so produced.
2. Background Information
A method for making treated silica fillers is taught in U.S. Pat. No. 2,993,809, issued July 25, 1961. Newly formed fumed silica, while at a temperature below 500.degree. C. is contacted with an organohydrolyzable silane. The silica is formed by combustion of silicon tetrachloride in the presence of hydrogen and oxygen, the amount of hydrogen being in excess so as to form sufficient water to convert all the silicon-bonded chlorine atoms in the silicon tetrachloride to silanol groups and to effect hydrolysis of the organohydrolyzable silane.
A method of treating reinforcing silica is taught in U.S. Pat. No. 3,024,126, issued Mar. 6, 1962. The method disperses a silica in an organic solvent and adds an organosilicon compound having hydroxyl or alkoxy radicals. There is also present a compound compatible with the solvent and selected from the group consisting of an amino compound having a basic dissociation constant in dilute solution in water of at least 10.sup.-7 at 25.degree. C., a quaternary hydrocarbon-substituted ammonium hydroxide, a salt of a phosphoric acid, and a salt of a carboxylic acid. Included are such materials as ammonia, certain organic amines, and silylorganic amines, such as [Me.sub.3 Si(CH.sub.2).sub.n ]CHNH.sub.2.
Silica is surface modified in U.S. Pat. No. 3,128,196, issued Apr. 7, 1964, by preferably fluidizing the silica and injecting a mixture of a liquid cyclic silane and a phenyl containing organosilicone compound having alkoxy groups.
Offenlegungsschrift No. 1,951,620, publication date of May 6, 1971, discloses a process for preparing hydrophobic, finely divided silicon dioxide. The process reacts silicon dioxide with from 1 to 10 percent by weight organoalkoxysilanes in the presence of amino compounds, the silane and amino compound being in the gaseous state at least a major portion of the reaction time, the reaction time typically being greater than 10 days.
An improved reinforcing silica filler is taught in U.S. Pat. No. 3,635,743, issued Jan. 18, 1972, as being made by first treating a reinforcing silica filler with ammonia and then with hexamethyldisilazane. The process claimed requires the silica to contain at least 0.2 weight percent of absorbed water at the beginning of treating. The ammonia can be replaced with primary amines and aminoxy compounds, and the hexamethyldisilazane with other silylamines and disilazanes.
British Application No. 1,371,218, published Oct. 23, 1974, and U.S. Pat. No. 4,164,509, issued Aug. 14, 1979, teach a process for hydrohobizing finely divided oxides. For example, a silicon oxide, having a water content of less than 1 percent by weight is intensively mixed, as in a fluidized bed, with a hydrolyzable metal or metalloid compound of the general formula M(OR).sub.4, where M can be silicon. The oxide can be treated with dry ammonia gas before, during, or after the admixture of the oxide with the ester compound.
A specially treated filler is used in a two-part room temperature vulcanizable silicone rubber composition described in U.S. Pat. No. 3,847,848, issued Nov. 12, 1974. The filler is prepared by intimately contacting silica filler, containing from 0.2 to 2.0 percent by weight of moieties selected from water, hydroxyl groups, and mixtures thereof, with a hydroxyl amine, a cyclic siloxane, and a silyl nitrogen compound of the formula (R.sup.4.sub.3 Si).sub.a X, where X is a nitrogen containing group. Preferably the treatment is carried out at a temperature of from 100.degree. to 180.degree. C., while the filler is being continuously agitated.
A process for hydrophobizing highly dispersed metal or metalloid oxides is described in U.S. Pat No. 3,920,865 issued Nov. 18, 1975. After drying the particles as a fluidized bed at a temperature of 600.degree. to 1000.degree. C. the dried particles are charged with at least one gas-phase organosilicon compound at a temperature of from 25.degree. to 650.degree. C., reacted at a temperature of from 35.degree. to 650.degree. C., and heated to from 125.degree. to 500.degree. C.
A filler which is subjected to mechanical stress and reacted with an organosilicon compound of the formula (R.sub.3 Si).sub.a Z' is described in U.S. Pat. No. 3,929,718, issued Dec. 30. 1975. R is a monovalent hydrocarbon radical or substituted monovalent hydrocarbon radical having up to 18 carbon atoms. Z'is a halide, hydrogen or a radical of the formula OH, OR', NR'X, ONR'.sub.2, SR', OOCR', O, N(CX), or S where R' is an alkyl or aryl radical and X is hydrogen or R'. Other organosilicon compounds which can be reacted are dimethyldichlorosilane, dimethyldiethoxysilane, diphenyldiethoxysilane, vinylmethyldimethoxysilane, octamethylcyclotetrasiloxane and/or dimethylpolysiloxanes having from 2 to 12 silicon atoms per molecule and having an Si-bonded hydroxyl group on each of the terminal units. The treated filler is used in organopolysiloxane elastomers.
U.S. Pat. No. 4,111,890, issued Sept. 5, 1978, contains a discussion of fillers suitable in curable organopolysiloxane compositions containing titanium esters. Included are treated fumed silicas, as well as other metallic oxides. It is preferred that the fillers be treated with organosilicon compounds to impart hydrophobic properties thereto. Organosilicon compounds which may be used are those having the general formula (R'''.sub.3 Si).sub.e Z or R'''.sub.e,SiZ'.sub.4 -.sub.e, wherein R''' which may be the same or different represents monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals, Z and Z' represent halogen, hydrogen, or a radical of the formula OR'''', --NR''''X', --ONR'''', --SR'''', or --OOCR'''' and when e is 2, then Z may also represent --O--, --NX'--, or --S--; R'''' is a hydrocarbon radical or a substituted hydrocarbon radical, X'' is hydrogen or has the same meaning as R''''; e is 1 or 2, and e' is 1, 2, or 3.
A method of surface treating particulate inorganic fillers with a surface-treating agent is taught in U.S. Pat. No. 4,454,288, issued June 12, 1984. The surface treating agent is an equilibrated reaction mixture consisting essentially of a specified ratio of R.sub.2 SiO.sub.2/2 units, RSiO.sub.3/2 units, and CH.sub.3 O.sub.1/2 radicals. When reinforcing silica filler is treated in such a method, it is useful with polydiorganosiloxanes curable to elastomers.
Blitz, Murthy, and Leyden teach in an article "Ammonia-Catalyzed Silylation Reactions Cab-O-Sil with Methoxymethylsilanes", JACS, 109,(23) 7141 (1987) that ammonia can be used to catalyze the reactions of methoxymethylsiloxanes with fumed silica in dry toluene medium. High temperature post-reaction curing is unnecessary for silylation to occur in either "dry" or "wet" Cab-O-Sil.